The present invention relates to a second harmonic generation method and apparatus, and more particularly, to a second harmonic generation method and apparatus capable of stabilizing an output.
In a recording/reproducing system used in an audio/video system such as a laser disk player, and an information recording device such as an optical magnetic drive, a laser having a linearly polarized stable output is required. Generally, the amplitude of an output laser beam can be stabilized easily by adjusting the input current of the laser diode, a light source, by the feedback control structure of the laser output. The output of an optical amplifying solid state laser system can be stabilized by controlling the laser output and amplification ratio. The laser device including an harmonic generating process by a non-linear birefringent crystalline material needs a complex feedback control structure.
A second harmonic generator using a pumping laser diode emitting blue-green light is a very useful light source for high-density optical magnetic recording. The second harmonic generation device in which a frequency doubling non-linear birefringent crystalline material is provided inside an internal resonator, is one laser device having the characteristic that the amplitude of the output laser is unstable. Accordingly, much research into second harmonic generating methods and the stabilization of the second harmonic output are underway.
Phase matching is a prerequisite for the effective and stable generation of the second harmonic.
Technology capable of realizing effective second harmonic generation with a low output was proposed in U.S. Pat Nos. 4,413,342 and 5,093,832. The former proposed a frequency doubling method of the internal resonator type. The laser resonator includes one pair of mirrors on which a coating layer of high reflectivity with respect to a fundamental wave is provided. In this method, an effective second harmonic generation can be realized with least loss by providing a non-linear birefringent crystalline material for frequency doubling inside the resonator to which a fundamental wave is injected at high strength. In the latter patent (U.S. Pat. No. 5,093,832), resonance occurs inside the frequency doubling birefringent crystalline structure, and second harmonic generation can be effectively realized by reinforcement of the fundamental wave in a resonator having such a structure. Here, a stable second harmonic was achieved by controlling the temperature of the frequency doubling non-linear birefringent crystalline material through the feedback control loop of the second harmonic.
Another temperature control method is shown in U.S. Pat. No. 3,858,056. In this method, the output of the laser separated by a beam splitter can be measured with a photo detector placed in the feedback control loop. In such a structure, although the second harmonic output has a maximum value at the correct temperature, an error signal is generated. Also, the error signal does not indicate which direction to adjust the temperature of the non-linear birefringent crystalline material. Accordingly, such a temperature control method is difficult to be apply because of the ambiguity of the error signal. Further, the error signal generated from the second harmonic divided by the beam splitter is not sensitive to the polarization change of the second harmonic, which is another problem of this method. That is, since the beam splitter has different reflectivities with respect to s-polarization and p-polarization, although the feedback circuit operates properly, it is difficult to stabilize the output of the second harmonic when the polarization change of the second harmonic is generated.
In any laser system in which the polarization state is one parameter, temperature control for stabilizing the output is required to be executed regardless of the polarization state of the second harmonic.